09-threads.md - add basic mutex and waiting demonstration

[taikedz]

Add a basic example extending the existing one:

• Uses the same sleepy function
• Adds a global integer counter
• Adds use of a mutex example
• Adds use of a wait group and thread pool for waiting for completions
• Uses defer on mutex and waitgroup as good practice demonstration
• Code is commented, minimally

Issues

• uses a sleep() to avoid reaching the waitAndWork(&wg) too soon. Feels like a hack, but not sure what else to do.

Tested with

• 0.13.0
• 0.14.0-dev.2441+3670910f2